Impregnation of wood chips with a cellulose protector followed by a soda-oxygen pulping stage

ABSTRACT

IMPREGNATING WOOD CHIPS WITH AN AQUEOUS SOLUTION OF AN INORGANIC SALT OF MAGNESIUM OR CALCIUM AS CELLULOSE PROTECTOR AND THEREAFTER PULPING THE WOOD CHIPS WITH SODIUM HYDROXIDE IN THE PRESENCE OF AN EXCESS OF OXYGEN.

United States Patent omee 3,695,994 Patented Oct. 3, 1972 3,695,994 IMPREGNATION F WOOD CHIPS WITH A CELLULOSE PROTECTOR FOLLOWED BY A SODA-OXYGEN PULPING STAGE Hans Edmund Worster, Richmond, British Columbia, and

Marian Franciszek Pudek, Coquitlam, British Columbia, Canada, assiguors to MacMillan Bloedel Limited, Vancouver, British Columbia, Canada No Drawing. Filed Apr. 13, 1970, Ser. No. 28,004

Int. Cl. D21c 9/10 US. Cl. 162-65 6 Claims ABSTRACT OF THE DISCLOSURE Impregnating wood chips with an aqueous solution of an inorganic salt of magnesium or calcium as cellulose protector and thereafter pulping the Wood chips with sodium hydroxide in the presence of an excess of oxygen.

BACKGROUND OF THE INVENTION (1) Field of the invention This invention relates to the manufacture of wood pulp and particularly to a soda pulping process carried out in the presence of oxygen.

(2) Description of the prior art It has been well established that sodium sulfide used in kraft pulping is responsible for air pollution with volatile sulfur compounds, such as hydrogen sulfide, methyl mercaptan, dimethyl sulfide and dimethyldisulfide. These compounds are emitted from the digester, spent liquor evaporators and the recovery furnace of a pulp mill. Similarly, in sulfide pulping, air pollution with sulfur dioxide and possibly also sulfur trioxide from sulfur burners, pulping liquor preparation systems, digesters and chemical recovery equipment is commonplace.

It has also been well known that soda pulping does not cause air pollution with volatile sulfur compounds because sodium hydroxide is the only pulping chemical used in this process. However, soda pulping has serious disadvantages compared to kraft pulping, such as lower pulp yield and slower pulping to a given degree of delignification. Higher chemical applications, longer reaction times and/or higher temperatures can, of course, compensate to a certain extent for the slower pulping of the soda process. However, it has not been possible to improve soda pulp yield by adjustment of process parameters. Chemicals which stabilize the carbohydrates against alkaline degradation, especially peeling, would be required if the pulp yield were to be increased to the level of the kraft process. These chemicals, e.g. borohydride, polysulfide, are much too expensive for use in a pulping process and would also introduce foreign elements (boron, sulfur) into the soda system.

Alkaline pulping in the presence of gaseous oxygen has been proposed in the past. However, it has never been a success commercially because of certain serious shortcomings. For instance, Marshall, US. Pat. 2,686,120, issued Aug. 10, 1954, describes single-stage alkaline pulping of lignocellulose in the presence of oxygen to produce pulp, vanillin and other oxidation products of lignin substances. However, the pulps obtained by that process are extremely low in tear factor indicating typical soda pulp quality. While no data on pulp yield obtained from wood chips is given in the above patent, it will be evident that it must be low due to cellulose degradation.

A process for pulping of bagasse, an annual plant, with ammonia and oxygen is described in US. Pat. 3,274,049

but the ammonia-oxygen process clearly would not be suitable for pulping of wood chips.

It has also been known to treat alkaline pulps with oxygen for the purposes of delignification and bleaching. For instance, US. Pat. No. 3,384,533 describes the effects of the action of oxygen on pulp in an alkaline medium in the presence of a catalyst, this catalyst being selected from barium carbonate, calcium carbonate, magnesium carbonate, inc carbonate, alkali metal borates and titanium dioxide. However, none of the examples of this patent describes the treatment of a soda pulp and, moreover, the introduction of the above catalysts into the system complicates the delignification, bleaching and recovery processes and significantly adds to production costs.

Yet another process for treating wood pulp with oxygen is described in US. Pat. No. 3,423,282 but in this instance the oxygen treatment is being used as a part of a twostage oxygen-chlorine treatment and is primarily a bleaching method rather than being directly related to pulping.

It is, therefore, the purpose of the present invention to provide a commercial acceptable single-stage soda-oxygen pulping process with improved pulp yield, while avoiding the above mentioned disadvantages of the prior art.

SUMMARY OF THE INVENTION According to this invention, it has been found that soda pulp can be obtained in excellent yield and quality if lignocellulosic material, such as wood chips, is first impregnated with a water soluble salt of a cellulose protector and the impregnated lignocellulose material is then pulped with sodium hydroxide in the presence of an excess of oxygen. As cellulose protector, a water-soluble inorganic magnesium or calcium salt, e.g. magnesium or calcium chloride is preferably used.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The impregnation conditions are not critical and the main consideration is that the cellulose protector must be thoroughly impregnated into the wood chips. For best results, an aqueous solution containing 5 to 20% by weight of the salt based on dry wood is used and an impregnation time of 10 to 30 minutes is usually quite adequate. The impregnation rate is, of course, greatly accelerated by application of pressure.

In a commercial operation in accordance with this invention, wood chips are fed into a digester and impregnated with an aqueous solution of one of the above mentioned cellulose protectors under pressure. After the impregnation of the chips has been completed, the remaining free impregnating solution is separated from the chips and brought to its original concentration prior to re-use. The impregnated chips are then pulped with sodium hydroxide under heat and pressure. The oxygen can be injected into the digester as soon as the alkaline solution is added, during the come-up period of the digester to maximum temperature, or even after the maximum temperature has been reached.

In order to assure maximum quality and yield we prefer to use a digesting temperature in the range of C. to 200 C. and an oxygen partial pressure of 100 to 180 p.s.1.g.

The application of the sodium hydroxide digestion liquor can also be maintained within the usual limits and depends on the degree of pulping that is desired.

Thus, it will be seen that the present invention provides a very simple and economical alkaline-oxygen pulping process and we have found that this new method gives improved pulp yields.

In addition, the alkaline pulping method provided by the present invention offers the following advantages over conventional kraft pulping:

(1) It does not cause air pollution with volatile sulfur compounds because of the complete absence of sulfur in the digestion system.

(2) Our process requires no black liquor oxidation and no equipment for collection and/or combustion of noncondensable sulfur gases.

(3) The process can tolerate direct contact evaporation and spent liquor because of the absence of sulfur.

(4) Our process does not require a high stack for the recovery furnace.

(5) It makes possible the utilization of stack gas heat because of the absence of corrosive gases, such as sulfur or hydrogen sulfide.

(6) Our process causes less corrosion than kraft pulping liquor and makes possible the use of materials other than stainless steel for the digester.

It will, therefore, be evident that our new process is applicable to existing mills or entirely new mills and is especially attractive in areas were kraft pulp mills are at present not permitted because of the problems of air pollution.

The present invention is illustrated by the following nonlimitative examples:

Example 1 A batch of Western hemlock chips with a bone dry wood substance content of approximately 50% was charged to a pressure reactor. A pulping liquor containing 15.5% so dium hydroxide expressed as sodium oxide and based on the dry weight of the wood chips was then added. The temperature of the liquor was then heated to 170 C. within 120 minutes by circulating it through a heat exchanger. Approximately 80 minutes after this temperature was reached, gaseous oxygen was injected into the pressure reactor to give a partial oxygen'pressure of 100 p.s.i.g. After another 50 minutes, pressure was relieved and the contents of the reactor was discharged.

The pulp prepared under the above conditions was washed, screened on an slot screen plate and tested for pulp yield and Kappa number. The following results were obtained:

Screened yield, percent on dry wood 48.7 Screen rejects, percent on dry wood 0.6 Kappa number 106.5

Example 2 In a pressure reactor, the hemlock chips described in Example 1 were impregnated with an aqueous solution containing 10% magnesium chloride based on dry wood for 20 minutes at room temperature and a nitrogen pressure of 100 p.s.i.g. The excess solution was then withdrawn. Subsequently, the chips were treated with alkali and oxygen under the conditions described in Example 1, except that 15.75% sodium oxide based on dry wood was applied.

4 The following results were obtained:

Screened pulp yield, percent on dry wood 50.7 Screen rejects, percent on dry wood 0.3 Kappa number 106 The ash content of this pulp was identical with that of the pulp described in Example 1.

These data indicate that the impregnation of chips with magnesium chloride prior to pulping with alkali and oxygen increased the screened pulp yield by 2% based on wood.

It is to be understood that the invention is not to be limited to the exact details of operation or the exact processes shown and described, as obvious modifications and equivalents will be apparent to those skilled in the art and the invention is to be limited only by the scope of the appended claims.

We claim:

1. In an alkaline pulping process wherein Wood chips are pulped with sodium hydroxide in the presence of an excess of oxygen, the improvement which comprises im pregnating wood chips with an aqueous solution of a water soluble inorganic salt of magnesium or calcium as cellulose protector, and thereafter pulping said impregnated wood chips with the sodium hydroxide in the presence of an excess of oxygen.

2. Process according to claim 1 wherein the cellulose protector is magnesium chloride or calcium chloride.

3. Process according to claim 2 wherein the wood chips are impregnated with an aqueous solution containing 5 to 20% by weight of the salt based on dry wood.

4. Process according to claim 3 wherein the impregnation time is 10 to 30 minutes.

5. Process according to claim 4 wherein the digestion is conducted at a temperature in the range of C. to 200 C.

6. Process according to claim 5 wherein an oxygen partial pressure of 100 to 180 p.s.i.g. is used.

References Cited UNITED STATES PATENTS 3,384,533 5/1968 Robert et al 162-65 2,686,120 8/1954 Marshall et a1 l62--65 X 2,811,518 10/1957 Mitchell et al 16265 X FOREIGN PATENTS 683,771 7/1969 Republic of South Africa.

OTHER REFERENCES Mellor, Inorganic and Theoretical Chemistry, vol. 4, p. 290.

S. LEON BASHORE, Primary Examiner A. L. CORBIN, Assistant Examiner US. Cl. X.R.

8-1 11; 162-89, Dig. 2 

